1. Field of the Invention
This invention relates to a triple mode filter and to a method of operating such a filter. In particular this invention relates to a filter having a cascade waveguide cavity resonating in first, second and third independent orthogonal modes simultaneously.
2. Description of the Prior Art
It is known to have a triple mode waveguide cavity filters. In COMSAT Technical Review, Volume 1, pages 21 to 42, published in the Fall of 1971, Atia and Williams suggested the possibility of cascading two triple mode waveguide cavities to realize a six pole elliptic filter function response. In theory, triple mode filters have an advantage over dual mode filters in that they produce economies in weight, volume and cost because of the realization of three electrical cavities in one physical cavity. However, previous triple mode filters have been unable to achieve acceptable results and, in particular, have failed to realize an elliptic function response. Also, previous triple mode filters have had input or output coupling means that are too complex or too heavy; or the filters have been too inefficient to compete with dual mode filters; or the intercavity coupling could not be adequately controlled. As previous triple mode filters did not produce the expected results, they are not widely used and the dual mode filter is now the dominant filter for use in satellites and multi-plexers. The communications satellite industry has long sought a solution to the problems related to previous triple mode filters.
It is an object of the present invention to provide a triple-mode filter that produces acceptable results and is lighter in weight and smaller in volume than comparable dual-mode filters.